The Spectrum+ is my preferred model (until I get a toast-rack), it’s keyboard is still bad, but not as bad as the rubber 48K. This one has an Issue 4S motherboard, which I wasn’t aware of, with a deep green solder mask.
As normal, the first task was to implement the composite mod with a capacitor – nothing new here and plenty of documentation already available. I also replaced the 7805 regulator with a drop-in switched mode version.
Initial testing with my diagnostic board showed that it was trying to boot but the screen was very dull and contained garbage (sorry for the poor quality photo). The reported symptom was that all the lower RAM was bad – not impossible with 4116 RAM, but highly unlikely.
My first guess was that one of the multiplexers was bad, which I have seen before, but testing all of the pins with my oscilloscope and everything looked ok. I then noticed a complete lack of 12v to the chips, given that the video encoder also needs 12v, this would explain why the screen was not behaving. This lead me to the TR4 and TR5 transistors.
The documentation I read pointed me in the direction of TR5, which I removed, but this had no effect on the issue. I replaced it with a compatible new transistor, but still no change to the fault.
I then replaced TR4 with a modern alternative. This had a catastrophic effect tripping my power supply immediately….
Note: on the 4S board the modern version of TR4 goes in alternative holes next to the original and is reversed (flat side to the left).
I decided that it was probably a short on the 12v line that had killed TR4 and the fault still existed. The only things that use 12v is the lower RAM and video encoder – so I removed these and replaced them with sockets.
This removed the short, and testing the removed RAM I had 4 bad chips, replacing the bad chips fixed the issue.
Thinking everything was ok I started to do some more in-depth testing, i.e. playing games. I then found that my divMMC refused to work on this Spectrum. Reading the documentation this pointed to a bad M1 line on the Z80, and testing with the oscilloscope confirmed that it was outputting noise rather than a valid signal. I decided to socket and replace the CPU.
With this completed and a new Z80 installed the divMMC worked as expected.. Another Spectrum+ returned to fully working order.